1. Field of the Invention
The present invention relates to an apparatus and a method for estimating a fading frequency of incoming received waves in a receiving station of a radio communication system or a radio transmission system.
2. Description of the Related Art
In receiving stations of mobile communication systems etc., the fading phenomenon that the amplitudes of received waves vary to a large extent occurs because having arrived via multiple paths whose individual transmission characteristics significantly vary, the received waves interfere with each other.
Such receiving stations reduce degradation in channel estimation accuracy by shortening the length of a pilot signal as the fading frequency increases, for example, the pilot signal used for the channel estimation and integrated. Also, They efficiently use radio frequencies and keep the number of terminals that can be accommodated per unit frequency by maintaining the transmitting power at a value that conforms to the fading frequency and the power of received waves.
FIG. 14 shows the configuration of an exemplary receiving station that is equipped with a fading frequency estimating apparatus.
As shown in FIG. 14, received waves are input to the input of a frequency converting unit 81 and the output of a local oscillator 82 is connected to a local frequency input of the frequency converting unit 81. The output of the frequency converting unit 81 is connected to the input of an A/D converter 83, and the output of the A/D converter 83 is connected to the input of a fading frequency estimating apparatus 84 and respective first inputs of a channel estimating unit 85 and a coherent detection unit 86. A fading frequency is obtained at the output of the fading frequency estimating apparatus 84, and the obtained fading frequency is supplied to a second input of the channel estimating unit 85. The output of the channel estimating unit 85 is connected to a second input of the coherent detection unit 86, and a detected signal is obtained at the output of the coherent detection unit 86.
The fading frequency estimating apparatus 84 is composed of the following components:                a pilot symbol averaging unit 91 that is disposed at the first stage;        a delay unit 92 whose input is connected to the output of the pilot symbol averaging unit 91;        an inner product calculating unit 93 whose first and second inputs are connected to the output of the delay unit 92 and the output of the pilot symbol averaging unit 91, respectively; and        a Cascade-connected averaging unit 94 and a judging unit 95 that are connected to the output of the inner product calculating unit 93.        
In the receiving station having the above configuration, the frequency converting unit 81 frequency-converts received waves on the basis of a local frequency signal of a prescribed frequency generated by the local oscillator 82 and thereby generates baseband signals corresponding to two respective orthogonal channels I and Q (in-phase component and quadrature component). The A/D converter 83 A/D-converts these baseband signals and thereby generates base band signals i and q in the digital domain.
In the fading frequency estimating apparatus 84, the pilot symbol averaging unit 91 receives two orthogonal partial baseband signals corresponding to known pilot symbols that have arrived as the received waves and generates noise-suppressed pilot signals i′ and q′ by averaging each of the in-phase component and the quadrature component.
The delay unit 92 delays the pilot symbols represented by the baseband signals i′ and q′ by a time equal to, for example, a cycle τ for transmission of the pilot symbols from a mobile station. Cooperating with the delay unit 92, the inner product calculating unit 93 calculates the inner product of two pilot symbols that are received in order with a delay that is equal to the cycle τ on the time axis.
In general, the inner product as used here is defined as the product of two amplitudes and a cosine value of a phase difference of a received pilot symbol and a pilot symbol that is received after a lapse of the time τ from the reception of the former pilot symbol, and hence means correlation between those pilot symbols.
The averaging unit 94 suppresses, through integration, a noise (white noise) component that has been superimposed on the pilot symbols in radio transmission paths formed between the receiving station and the mobile station and whose amplitude average value can be regarded as 0.
The judging unit 95 has a time correlation table that relates to, for example, propagation, satisfying the following requirement, of a radio-frequency signal in a city area and gives, for respective discrete fading frequencies, time correlation values ρ(τ) that are given by a Bessel function of the first kind J0(·) (Equation (1)) for the cycle τ and the fading frequency (in general, proportional to the speed v of a mobile station and inversely proportional to the wavelength λ of a received wave).
For received waves of a mobile station, Equation (1) holds in the case where transmission waves that come from a base station and are reflected by scatterers around the mobile station are regarded statistically as incident on the mobile station at approximately the same power for all directions.
For received waves of a base station, Equation (1) holds in the case where transmission waves that are emitted from a mobile station to all directions and are reflected by scatterers around the mobile station are regarded statistically as incident on the base station at approximately the same power.ρ(τ)=J0(2πfτ)  (1)
Further, the judging unit 95 refers to the time correlation table on the basis of the inner product value that is supplied from the averaging unit 94 as described above, and thereby determines a time correlation value ρ(τ) that is smallest (or having a smallest deviation from the inner product value) among time correlation values larger than the inner product value. The judging unit 95 estimates the fading frequency f of the received waves to be a value corresponding to the determined time correlation value ρ(τ). In the following description, processing of estimating a fading frequency f by referring to the time correlation table will be referred to as threshold judgment.
The channel estimating unit 85 performs channel estimation for determining transmission characteristics of the above-mentioned radio transmission channels on the basis of deviations of the above-mentioned pilot symbols among the sequences of symbols represented by the baseband signals i and q that are generated by the A/D converter 83. The number of pilot symbols to be referred to in a process of such channel estimation is set smaller as the fading frequency f increases.
Therefore, even if the fading frequency is high, the accuracy of channel estimation is kept higher than in a case that channel estimation is performed on the basis of a constant number of pilot symbols.
The coherent detection unit 86 generates a detected signal by performing coherent detection on the base band signals i and q under the above channel estimation.
Therefore, even if the fading frequency varies widely, the fading frequency is estimated accurately and hence a high transmission quality is maintained.
Prior art techniques relating to the invention are as follows:
(1) A fading pitch detecting apparatus and a portable data terminal using the fading pitch detecting apparatus both for generating a composite signal by combining signals having phase differences occurring in each multiple path and output from a plurality of despreading devices that are connected to a common receiving system and perform despreading for each multiple path and for detecting a fading pitch on the basis of a magnitude relationship between an autocorrelation value of the composite signal and a predetermined threshold value (disclosed in JP, (A), No. 2001-223671);
(2) A frequency offset correcting apparatus for estimating a fading signal sequentially by detecting a correlation signal which is obtained by digitizing a received baseband signal to obtain a digital baseband signal and despreading the digital baseband signal by use of a spreading signal that is shifted in time, and for taking correlation between a correlation value signal and each of a transmitted signal and an estimated signal on the basis of a signal representing a result of the detection (disclosed in Japanese Patent No. 2,705,613);
(3) A CDMA receiver in which a channel estimation value is generated for each symbol by adding each of pilot symbols P0 to PS to the adjacent (preceding and following) pilot symbols with in-phase adders, a symbol-based channel estimation value is generated immediately before and after an interval when no pilot signals are transmitted, channel compensation and calculation of slot-based channel estimation values are performed by multiplying data symbols by complex conjugates of the above channel estimation values, and weighting coefficients used for the above in-phase addition are controlled by selecting slot-based or symbol-based channel estimation values in accordance with a fading frequency or reception quality (disclosed in JP, (A), No. 2003-69451);
(4) A fading frequency estimating circuit and a CDMA receiving apparatus having that circuit in which noise resistance is increased by determining a fading frequency estimation value by using a slot time correlation value corresponding to a differentially coherent detection output with a minimum slot delay as a denominator and another slot time correlation value as a numerator (an inner product value is normalized prior to its magnitude judgment with respect to a threshold value) and weighted combining etc. are controlled by inputting the fading frequency estimation value to a searcher unit, a weighting coefficients control unit, and an SIR estimating unit (disclosed in JP, (A), No. 2001-358621); and
(5) A channel estimating apparatus and method, a demodulation apparatus and method, and a fading frequency determining apparatus and method in which weighting coefficients are varied in accordance with a fading frequency that is determined according to inner product values of pilot symbols (disclosed in WO 00/60761).
In the above conventional examples, the shorter the interval on the time axis between two pilot symbols determining an inner product value to be judged, the lower the accuracy of the threshold judgment. Therefore, a low fading frequency is determined by a threshold judgment with a longer interval between two pilot symbols, which requires a delay circuit that provides a long delay time.
However, in general, the oscillation frequency of the local oscillator 82 provided in the receiving station has a deviation (hereinafter referred to as first deviation) from the oscillation frequency of a local oscillator of a transmitting station and the frequencies of receiving waves may have deviations (hereinafter referred to as second deviations) that are caused by the Doppler effect in propagation paths.
Therefore, it is needed to determine an inner product value to be subjected to the threshold judgment after correction of phase errors in pilot symbols due to the first and second deviations d. However, in practice, it is difficult to extract the first and second deviations from the fading component since they are included in a fading component of incoming received waves.
On the other hand, in recent years there has been a strong demand for mobile communication systems to improve their performance and added values by utilizing highly advanced digital signal processing etc. It is strongly desired that a fading frequency of received waves be determined accurately at a low cost.